Acid catalysts find use in a broad range of industrially important processes including but not limited to cracking, isomerization, alkylation, acylation, and polymerization. Many of these processes are significantly advanced in terms of their chemistry; however, room for improvement still exists. In particular, one of the main limitations of acid catalysts is that many operate efficiently only under homogeneous conditions. As a result, in a majority of instances it becomes difficult if not impossible to separate the catalyst from the reaction mixture; thus, catalyst is consumed upon workup of the reaction mixture and isolation of the product. From this standpoint, these processes are wasteful in terms of materials and are not environmentally beneficial. A need exists for acid catalysts that possess a high degree of efficacy while operating under heterogeneous conditions as to preclude contamination of the reaction mixture with catalyst residues and increase overall utilization of the catalyst. This invention is aimed at these goals in addition to others.
The need for such heterogeneous acid catalysts is best illustrated in the field of cationic polymerization; in particular the production of isobutylene based polymers and most specifically polyisobutylene (PIB) and butyl rubber. The molecular weight (MW) of polymers produced by cationic methodologies is limited by a process known as chain transfer (CT). In cationic olefin polymerization, CT involves β-deprotonation of the propagating carbocationic chain end of growing polymer with concomitant formation of a new carbocation capable of initiating further growth. CT is suppressed in relation to propagation by conducting polymerizations at reduced temperatures as the activation energy for the former process greatly exceeds that for the later. CT can also be reduced by using high monomer concentration, polar solvents, and weakly coordinating anions (WCAs).
Aluminum chloride (AlCl3) is the main acid catalyst (coinitiator) used in the production of butyl rubber despite numerous drawbacks. Under heterogeneous conditions, AlCl3 is not an efficient acid coinitiator for isobutylene polymerization. Therefore, polymerization is conducted at ˜−100° C. under slurry conditions using methyl chloride (CH3Cl) as a polar solvent for AlCl3, a diluent for the monomers/polymer, and as a heat sink. Despite the polar environment, low temperatures are still required for the production of high MW polymer (≦−95° C. for Mv>1×106 g·mol−1) as the anions derived from AlCl3 promote CT and thus polymerization is costly in terms of refrigeration. CH3Cl is toxic and requires special handling, further detracting from the utility of this process. Moreover, AlCl3 has very limited solubility in CH3Cl and accurate determination of the concentration is difficult. This complicates metering a given quantity of AlCl3 into the reactor and limits control over the polymerization process. Additionally, recycled CH3Cl must be freed of monomer impurities prior to reuse in the preparation of fresh AlCl3 coinitiator solution as they oligomerize and coat the AlCl3 preventing it from dissolving. Since the butyl product is insoluble in CH3Cl, problems with reactor fouling arise requiring periodic shutdown and cleaning cycles leading to productivity losses and increased operation costs. Post polymerization functionalization (e.g. halogenation) of butyl also requires removal of CH3Cl and subsequent dissolution into an aliphatic hydrocarbon or other appropriate solvent increasing the number of processing steps. Impurities arising from AlCl3 can interfere with such reactions. Finally, AlCl3 remains trapped within the polymerization mixture necessitating deashing steps that destroy AlCl3 and result in additional processing steps, energy consumption, and waste generation.
Numerous initiator systems involving homogeneous coinitiators have been devised in an attempt to overcome the negative aspects of AlCl3 based polymerization. These systems can be categorized into six main subsets.
1. Initiator systems that employ a halogen bearing aluminum Lewis acid coinitiator exhibiting enhanced solubility characteristics.
2. Those involving soluble alkyl, alkoxy, amino, or oxy substituted Lewis acids in conjunction with other Lewis acids.
3. Those that employ an aluminum Lewis acid coinitiator that bears organic substituents, in particular trialkylaluminum compounds and diethylaluminum chloride (Et2AlCl), in conjunction with an initiator component other than adventitious moisture.
4. Initiator systems containing WCAs based on alkylaluminoxanes or perfluoroarylated Lewis acids (PFLAs).
5. Initiator systems based on organo zinc halide coinitiators in conjunction with carbocation synthons.
6. Physical initiator systems (γ-radiation.).
One of the first effective homogenous systems explored involved the use of aluminum bromide (AlBr3) in place of AlCl3. In comparison to AlCl3, AlBr3 exhibits greater solubility in a wider range of solvents including hydrocarbons and polymerizations using the latter can be conducted under homogeneous conditions in nonpolar media avoiding requisite use of toxic CH3Cl. Such polymerizations also give rise to higher MW butyl at a given reaction temperature in comparison to AlCl3 despite being conducted under nonpolar reaction conditions and thus ease the refrigeration requirements necessary to produce a specific grade of polymer. Moreover, since polymerization is conducted in solution, problems with reactor fouling are avoided. Of the halogen bearing aluminum coinitiators, ethylaluminum dichloride (EtAlCl2) is purported to yield highest MW polymers in CH3Cl. EtAlCl2 can also function in hydrocarbons and thus has many of the same benefits as AlBr3. In particular, aqua adducts of alkylaluminum dihalides (e.g. EtAlCl2.OH2) are highly active for the production of high MW butyl at elevated temperatures in nonpolar solvents.
A large number of homogeneous initiator systems have been developed that involve the combination of multiple Lewis acids. These systems are interrelated in that most contain a Lewis acid component that bears either an alkyl group or heteroatom containing substituent (e.g. alkoxide, amide, or oxide) to improve the solubility and activity of more traditional halogen substituted Lewis acids. Kraus and Young/Kellog were the first to report the use of alkyl and alkoxy/oxy substituted Lewis acids (respectively) as adjuvants to improve the solubility and hence activity of acid metal halide coinitiators in the polymerization of isobutylene {e.g Me2AlCl+MeAlCl2; Ti(OBu)4+AlCl3}. Even though these polymerizations were conducted in CH3Cl at low temperatures (˜−78° C.) MWs were reported to be low. As will be seen this is surprising since subsequent disclosures in the patent literature, in some cases using almost identical components under similar reaction conditions (e.g. Parker and coworkers {Et2AlCl+EtAlCl2}; Strohmayer and coworkers {Et3Al+TiCl4}), give rise to high MW products. The cause of such discrepancies may be attributed to the use of dry ice as an internal cooling agent as CO2 is known to react with alkyl aluminum compounds, key components of these initiator systems. When the use of dry ice as an internal cooling agent is omitted a 9:1 molar mixture of Et2AlCl:EtAlCl2 gives rise to high MW butyl in hexane solution at temperatures in the vicinity of −70° C. Of further interest, it was later discovered that metal alkoxides, metal amides, and mixed metal oxide-metal alkoxides in conjunction with BF3 are highly active for the production of high MW butyl in aliphatic solvents at high temperatures. For example, Group 8, 9, and 10 metal alkoxides {e.g., Fe(OBu)3} in conjunction with BF3 give rise to good yields of high MW PIB/butyl at elevated temperatures in hexanes. In a similar vein, alkoxy aluminum and titanium halides {e.g., ClAl(O-sec-Bu)2; Cl3TiOBu} also form very active initiator systems in conjunction with BF3 that yield high MW polymers at elevated temperatures in nonpolar media. Mixed metal oxide-metal alkoxides {e.g., Zn[OAl(OEt)2]2} in conjunction with BF3 were also found to exhibit similar behavior. In an analogous manner, alkoxy aluminum halides {e.g., Cl2AlOMe}, and alkoxy alkyl aluminum halides {e.g., EtAl(OEt)Cl} in conjunction with a wide variety of halogen bearing Lewis acids (e.g., TiCl4) give rise to high MW butyl at elevated temperatures although polymerization appears to require CH3Cl. Likewise, metal (Zn, Al, Ti, Sn, Si, Zr, etc.) amide bearing Lewis acids {e.g., Al(NEt2)3} with BF3 exhibit high activity for the production of high MW PIB/butyl in nonpolar solvents at elevated polymerization temperatures.
A great deal of research has been conducted on the use of Et2AlCl as a coinitiator in the polymerization of isobutylene. Unlike EtAlCl2, Et2AlCl requires purposeful addition of an initiator component. Initiator components that have been found to be useful in combination with Et2AlCl include organic halide carbocation synthons (e.g. t-butyl chloride), hydrohalogen acids, halogens/interhalogens, electron acceptors (e.g. tetracyanoethylene), sulfur oxides, alkali and alkaline earth metal salts, as well as alkyl metal/semimetal halides, metal alkoxy halides, and metal oxy halides (e.g. MeSiCl3, Cl3TiOBu, ZrOCl2). These systems all give rise to high MW grades of PIB/butyl, but from the published data, use of CH3Cl is a required in all cases. Triorganoaluminum compounds are active coinitiators for the production of high MW PIB at elevated temperatures in conjunction with initiators ranging from organic halide carbocation synthons (e.g. t-butyl chloride), hydrohalogen acids (e.g. HCl), and halogens/interhalogens (e.g. Cl2) when used in polar solvents (e.g. CH3Cl). The MWs of polymers produced by these systems for a given initiator are lower than those yielded by the corresponding Et2AlCl system at a specific polymerization temperature.
Initiator systems based on alkyl and aryl zinc halide cointitiators (e.g. EtZnCl) in conjunction with carbocation synthons (e.g. t-butyl Cl) afford high MW PIBs at elevated temperatures. Despite these benefits, such systems have little utility in that they only operate efficiently in polar solvents (e.g. CH2Cl2) and at temperatures>−35° C.
Physical initiator systems have also been developed for the polymerization of isobutylene. For example, γ-radiation provides highest MW polymers at a given temperature but requires monomer of such high purity as to be impractical to conduct on a commercial setting. The requisite use of high energy radiation also detracts from the utility of this process.
Recently a great deal of research has been conducted on initiator systems that contain WCAs. These systems can be classified into five distinct groups dependent on the mode of initiation and identity of the initiator system components.    1. Systems that give rise to protic initiation from Brönsted acids generated in situ by reaction of PFLAs {e.g. B(C6F5)3; 1,2-C6F4[B(C6F5)2]2} or their salts {e.g. [Li]+ [B(C6F5)4]−} with adventitious moisture.    2. Those systems that generate initiating carbocations from reaction of PLFAs or their salts with carbocation synthons (e.g. t-BuCl).    3. III-defined initiation processes involving in situ formation of silylium tetrakis(pentafluorophenyl)borate.    4. Direct or indirect (protic) initiation processes involving metal cations derived from transition metal complexes and PFLAs.    5. Systems derived from methylaluminoxane (MAO) in conjunction with an initiator.
With the exception of certain PFLA derived salts {i.e. trityl tetrakis(pentafluorophenyl)borate, [Ph3C]+[B(C6F5)4]−} these systems are capable of producing high MW polymer at elevated reaction temperatures. The primary drawbacks to the first four methods are the expense of the homogenous initiator components and their sensitivity to minute traces of impurities (e.g. moisture). Moreover, in some cases {B(C6F5)3+H2O; B(C6F5)3+2-chloro-2-phenyl propane (cumyl chloride)} polar solvents (e.g. CH3Cl) are required to facilitate polymerization.
Systems based on MAO are superior to those that use PFLAs from a cost standpoint; however, active initiators appear to be limited to halogen bearing carbocation synthetic equivalents and adventitious moisture (see below) under nonpolar reaction conditions despite claims that other carbocation synthons containing groups that are typically ionizable (e.g. acyl halides and 3° ethers) are active as well. Moreover, these systems have limited activity at high reaction temperatures under nonpolar conditions and due to their homogeneous nature deashing steps are required. It should be noted that, in the systems Et2AlCl+EtAlCl2+MAO and EtAlCl2+MAO, the aluminoxane appears to function primarily as a scavenger of moisture and not necessarily as an actual coinitiator. This is most evident for the EtAlCl2+MAO system where polymer MW increases while yield decreases and molecular weight distribution (MWD) narrows with increasing [MAO]. Such behavior is indicative that MAO is either scavenging the initiator (e.g. adventitious moisture present as EtAlCl2.OH2) and/or chain transfer agents (e.g. H2O) thus lowering their overall concentration.
Despite the enormous amount of research it is evident that even the best homogeneous initiator systems still suffer in that deashing steps are required for removal of spent initiator components. As a result, a great deal of work has focused on developing initiator systems that use heterogeneous coinitiators. These systems can be grouped into seven main classes.
1. Silica and alumina supported AlCl3.
2. Acidic inorganic solids (e.g. MgCl2, clays, molecular sieves).
3. Inorganic solids containing intercalated Lewis acids.
4. Complex acidic solids from reaction of Al(O-sec-Bu)3, BF3, and TiCl4.
5. Metal triflates, perchlorates and their supported analogs.
6. Mixed Lewis acids supported on inorganic oxides.
7. Polypropylene (PP) supported Al and B containing Lewis acids.
A tremendous amount of research has been conducted on silica and alumina supported AlCl3 as heterogeneous Lewis acids. Two main approaches have been explored in an attempt to yield a support material bearing —OAlCl2 groups. In one, AlCl3 is reacted with a support material by dry mixing/pelletization, reaction under melt conditions, vapor phase reaction (e.g., sublimation), and solution reaction. A second approach involves reaction of alkylaluminum dihalides with the support. None of these materials are capable of producing high MW polymers at elevated temperatures.
Acid treated clays and activated 5-A molecular sieves have been explored as solid acid catalysts for cationic polymerization. The former were used in the preparation of low molecular weight styrenic resins and no information as to their utility in isobutylene polymerization was provided whereas the later are reported to yield low molecular weight PIBs at elevated temperatures in neat monomer after long reaction times. Thus, these materials are not useful for the preparation of high MW grades of PIB or butyl.
Freshly milled CdCl2 layer structure dihalides (e.g., MgCl2) are active coinitiators for IB yielding high MW polymers at elevated temperatures. Polar solvents (e.g. CH3Cl) and careful manipulation of the moisture level are required for high activity thus limiting the utility of these systems. It was suggested that Mg2+ generated during the milling process reacts with adventitious moisture to form a strong Brönsted acid that ultimately initiates polymerization.
Supported Lewis acids have been made by intercalating them within an inorganic metal dihalide (e.g., MgCl2). The intercalation process involves application of a hydrocarbon soluble porogen (e.g., adamantane) in conjunction with the Lewis acid which are mixed together with the inorganic halide in the solid state followed by selective solvent extraction of the porogen. The resultant materials are active for polymerization of IB but yield only low MW materials at high temperatures and are not useful in the preparation of high MW PIB or butyl.
A complex solid Lewis acid coinitiator synthesized from Al(O-sec-Bu)3, BF3, and TiCl4 is active for polymerization of IB to high MWs at elevated temperatures in aliphatic solvents. This was formed by initial reaction of Al(O-sec-Bu)3 with BF3 to form a precipitate that was then subsequently treated with TiCl4 just prior to polymerization. Both the Al(O-sec-Bu)3/BF3 precipitate and its reaction product with TiCl4 are thermally unstable and degrade with time even at temperatures <0° C. Both polymerization rate and polymer MW are adversely affected by aging of these materials limiting the usefulness of this system.
A variety of unsupported metal perchlorates and triflates have been explored for cationic polymerization under heterogeneous conditions. Of these only Mg(ClO4)2 was shown to give rise to high MW PIB at elevated temperatures (i.e. 0° C.) in neat monomer, albeit in low yields. Evidence for initiation by direct addition of monomer to exposed metal cations was gathered. Supported analogs of these materials have been described and can be synthesized by reacting a supported metal halide precursor (e.g., —OAlCl2) with an appropriate Brönsted acid (e.g., CF3SO3H) to effect transesterification and formation of the corresponding metal triflate or perchlorate {e.g., —OAl(CF3SO3)2}. These materials give rise to high yields of low MW PIBs exhibiting broad MWDs at elevated temperatures. Therefore, from the published data these materials are not suitable for the preparation of high MW grades of PIB or butyl.
Heterogeneous acid catalysts containing a mixture of weak and strong Lewis acid sites were made by first reacting alkyl substituted strong (e.g., Et2AlCl) and weak (e.g., MgBu2) Lewis acids with an inorganic support bearing hydroxyl groups. Any residual alkyl moieties of the supported acid sites were then converted to halogen substituents using halogens or alkyl halides to effect ligand exchange and yield a solid bearing strong (e.g., —OAlCl2) and weak (e.g., —OMgCl) acid metal halides. These materials are active for the polymerization of isobutylene at elevated temperatures in nonpolar solvents; however, even though Mw is high MWD is abnormally broad limiting the utility of these systems.
Polypropylene (PP) and polybutene-1 (PB) substituted with —OAlCl2, —OAlClEt, —O(H)—BF3, and —OBF2 groups are active Lewis acid coinitiators for isobutylene polymerization. PP and PB substituted with —OAlCl2, —OAlClEt, —O(H)—BF3 were made by reaction of hydroxyl functionalized PP and PB with EtAlCl2, Et2AlCl, and BF3 (respectively) whereas —OBF2 substituted PP was made by conversion of hydroxylated PP to a lithium alkoxide analog followed by subsequent reaction with BF3. The —OAlCl2, —OAlClEt variants produce good yields of high MW polymers at elevated temperatures in polar (e.g. CH2Cl2) solvents. In nonpolar solvents they only produce polymers with low MWs. Polymers bearing —O(H)—BF3 and —OBF2 groups produce only low MW polymers. These coinitiators can be reused several times without apparent loss of activity. Although —OAlCl2, —OAlClEt functionalized PP are the most promising heterogeneous Lewis acid coinitiators disclosed to date their utility is hampered by the fact that they require polar solvents for the preparation of high MW grades of polymer and because the hydroxyl functionalized polymer support is not readily available and requires special/costly synthesis.
While numerous initiator systems have been researched and developed for the preparation of high MW polymers (such as, for example and in particular, butyl rubber and PIB) at elevated reaction temperatures under cationic conditions, none of these initiator systems address all, or even most, of the aforementioned deficiencies inherent in cationic polymerization. More particularly, no heterogeneous initiator system that is easy to manufacture and has convenient shelf stability has been known to produce polymers higher in MW than the present invention using the cationic polymerization of olefin monomers at elevated reaction temperatures (i.e. those temperatures above about −100° C.) without requiring the use of chlorinated solvents. Deleting the use of such chlorinated solvents will undoubtedly result in providing substantial savings in terms of both monetary costs as well as energy, and will reduce the impact on the use of such deleterious compounds on the environment.
Thus, there exists a need to provide a initiating system at a low cost that is capable of producing a polymer from one or more olefin monomers with a MW at least equal to and, in most cases, exceeding that which can be obtained from the aforementioned systems at a given temperature in the absence of chlorinated solvents. There is further a need to provide an initiating system in a form that is conducive to multiple batch and/or continuous polymerization processes and can be readily isolated from the reaction medium or products in order to minimize product purification steps and to minimize waste while maximizing economic and environmental benefits.